A polyacetal resin is excellent in mechanical property, fatigue resistance, friction or abrasion resistance, chemical resistance, and moldability. Therefore, the polyacetal resin has been widely utilized in various fields such as an automotive part, an electric or electronic device part, other precision machinery part, an architectural or pipeline part, a household utensil or cosmetic article part, or a medical device part. While, along with expansion or diversification in application, the polyacetal resin tends to be required highly developed quality. Characteristics (or properties) required for the polyacetal resin includes characteristics that mechanical strength in a process step such as an extruding step or a molding step is not deteriorated, that deposit to a metal mold (or mold deposit) is not generated, that mechanical property under a long-term heating condition (heat aging) is adversely affected, and that incomplete molding such as solver streak or void is not found in a shaped article. The degradation of the polymer upon heating is one of the important factors among these phenomena. In particular, the polyacetal resin is inherently unstable in an oxidative atmosphere at an elevated temperature or in an acidic or alkaline environment because of its chemical structure. Therefore, the essential need that must be fulfilled for a polyacetal resin is that of insuring high thermal stability and minimal emission (or generation) of formaldehyde in the course of processing and from shaped articles. Formaldehyde is chemically active and ready to be oxidized to formic acid to thereby adversely affect the heat resistance of resin and, when the resin is used as electric or electronic parts, cause corrosion of metallic contacts or their discoloration due to organic deposits, resulting in contact errors. Furthermore, formaldehyde as such contaminates the working environment for parts assembling and the ecology in the field of use of end products.
In order to stabilize chemically active terminals, the following methods are known: for a homopolymer, a method of esterifying by acetylation or other means of the terminal of the polymer; and for a copolymer, a method of copolymerizing trioxane and a monomer having an adjacent carbon bond (e.g., a cyclic ether or a cyclic formal), and then decomposing and removing unstable terminal sites to make the unstable terminal sites stable (or inactive) terminal sites. However, the main chain part of the polymer also causes cleavage (or fission) decomposition in a heating process. Inhibiting such a decomposition cannot be overcome by only the above-mentioned treatment, and practically, it is considered that addition of an antioxidant and other stabilizers is required for such inhibition.
However, even in the case blending these stabilizers, it is difficult to completely inhibit decomposition (or degradation) of the polyacetal resin. In practice, upon melt processing in an extruding step or a molding step for preparing a composition, the polyacetal resin undergoes an action of heat or oxygen inside of a cylinder of an extruder or a molding machine, thereby decomposing the main chain or generating formaldehyde from an insufficiently stabilized terminal thereof, as a result, working environment is worsen in a extruding and molding process. Moreover, in the case carrying out molding for a long period, a finely powdered substance or a tar-like substance is deposited on a metal mold (mold deposit), thereby decreasing working efficiency. In addition, the mold deposit is one of the ultimate causes in deteriorating the surface condition of the shaped article. Further, the polymer decomposition causes deterioration in mechanical strength of the resin, and discoloration thereof. From such a viewpoint, a good deal of effort is continued for ensuring more effective stabilizing formulation (or recipe) about the polyacetal resin.
As the antioxidant added to the polyacetal resin, a phenol-series (phenolic) compound having steric hindrance (hindered phenol), and an amine compound having steric hindrance (hindered amine) have been known. As other stabilizers, a specific nitrogen-containing compound such as melamine or a derivative thereof, a polyamide, or a polyacrylamide derivative, an amidine compound, an alkaline metal hydroxide, an alkaline earth metal hydroxide, and an organic or inorganic acid salt have been used. Moreover, antioxidants are generally used in combination with other stabilizers. However, even when such an additive(s) is/are used, it is difficult to give the polyacetal resin high stability.
Japanese Patent Application Laid-Open No. 59646/1977 (JP-52-59646A) discloses that a polyacetal resin composition obtained by adding an antioxidant, an alkyleneurethane and urea to a polyacetal copolymer is improved in stability for heat and oxidative atmosphere, and does not cause coloration. However, it is difficult to ensure significant inhibition of formaldehyde emission by only adding the antioxidant and urea to the polyacetal copolymer.
Japanese Patent Application Laid-Open No. 145245/1986 (JP-61-145245A) discloses a molding composition which is prepared by blending a small amount of an ionic salt of a low molecular weight copolymer obtained from an α-olefin and an α,β-ethylene-type (ethylenic) unsaturated carboxylic acid to an acetal polymer for improving thermal stability of a polyacetal. Moreover, this document discloses that a cyanoguanidine, a triazine, or the like is used as an amidine-series stabilizer.
Japanese Patent Application Laid-Open No. 260949/1988 (JP-63-260949A) discloses a polyacetal composition for molding obtained by adding an additive such as a hindered phenol, a metal salt of a hydroxycarboxylic acid, a lubricant, a nitrogen-containing heat stabilizer (melamine, an amidine compound such as cyanoguanidine), a nucleating agent, or an antistatic agent to a polyacetal resin. In this document, these additives contribute to improve resistance for yellow discoloration in heat aging, mechanical property, processing aptitude, stability for ultraviolet ray, and resistance for storage of static electricity.
According to these documents, heat stability, mechanical property, or molding workability may be improved, however, it is difficult to efficiently reduce formaldehyde emission. In addition, it is also difficult to add large amounts of the additives to the composition because these bleed (or leach) out from a shaped article formed with the composition.
On the other hand, it is also conceivable to combine a polyacetal resin with other material(s) for improving property of the polyacetal resin. However, the polyacetal resin has a low affinity or compatibility to other material(s) due to highly crystallinity thereof. For example, the polyacetal resin is known to have compatibility to only a novolak phenol resin among various polymers (KOBUNSHI RONBUNSHU (Japanese Journal of Polymer Science and Technology), Vol. 48, No. 7, pp. 443 to 447 (July, 1991)). Japanese Patent Application Laid-Open No. 98039/1993 (JP-5-98039A) discloses a biaxially stretched polyoxymethylene film composition comprising 50 to 99 parts by weight of a polyoxymethylene polymer and 50 to 1 parts by weight of a novolak phenol resin. This document describes that the composition comprising the polyoxymethylene polymer and the novolak phenol resin at a weight ratio [the former/the latter] of 99/1 has insufficiently stretching stability. However, the relative large amount of the phenol resin in this composition makes it difficult to express the properties of the polyoxymethylene polymer effectively.
Japanese Patent Application Laid-Open No. 72830/2001 (JP-2001-72830A) discloses a polyacetal resin composition comprising a polyacetal resin and a flame retardant, wherein the flame retardant comprises a phosphorus-containing compound and an aromatic compound which accelerates flame retardation in association with the phosphorus-containing compound. The aromatic compound includes a phenol novolak resin, a phenol aralkyl resin, an aromatic vinyl resin, or others. This literature also discloses that the proportion of the phosphorus-containing compound is 1 to 250 parts by weight per 100 parts by weight of the aromatic compound and the total amount of the phosphorus-containing compound and the aromatic compound is 1 to 100 parts by weight per 100 parts by weight of the polyacetal resin.